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  Java SE基础篇(三)
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      <h2 id="数据输入"><a href="#数据输入" class="headerlink" title="数据输入"></a>数据输入</h2><h3 id="Scanner使用的基本步骤"><a href="#Scanner使用的基本步骤" class="headerlink" title="Scanner使用的基本步骤"></a>Scanner使用的基本步骤</h3><h4 id="①导包"><a href="#①导包" class="headerlink" title="①导包"></a>①导包</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> java.util.Scanner;</span><br><span class="line"><span class="comment">//导包必须出现在类定义上面</span></span><br></pre></td></tr></table></figure>
<h4 id="②创建对象"><a href="#②创建对象" class="headerlink" title="②创建对象"></a>②创建对象</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Scanner sc=<span class="keyword">new</span> Scanner(System.in);</span><br></pre></td></tr></table></figure>

<h4 id="③接收数据"><a href="#③接收数据" class="headerlink" title="③接收数据"></a>③接收数据</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> i = sc.nextInt();</span><br></pre></td></tr></table></figure>

<p>案例：从键盘获取三个数，用三元运算符来比较最大值，输出结果</p>
<a id="more"></a>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">Scanner in=<span class="keyword">new</span> Scanner(System.in);</span><br><span class="line"><span class="keyword">int</span> num[] = <span class="keyword">new</span> <span class="keyword">int</span>[<span class="number">3</span>];</span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">0</span>;i&lt;=<span class="number">2</span>;i++)&#123;</span><br><span class="line">    System.out.println(<span class="string">"请输入第"</span>+(i+<span class="number">1</span>)+<span class="string">"个数"</span>);</span><br><span class="line">     num[i]=in.nextInt();</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">int</span> temp=num[<span class="number">0</span>]&gt;num[<span class="number">1</span>]?num[<span class="number">0</span>]:num[<span class="number">1</span>];</span><br><span class="line"><span class="keyword">int</span> max=temp&gt;num[<span class="number">2</span>]?temp:num[<span class="number">2</span>];</span><br><span class="line">System.out.println(<span class="string">"最大值为："</span>+max);</span><br></pre></td></tr></table></figure>

<h2 id="顺序结构"><a href="#顺序结构" class="headerlink" title="顺序结构"></a>顺序结构</h2><p>最基本的流程控制，没有特定的语法结构，按照代码的先后顺序依次执行。</p>
<h2 id="if语句"><a href="#if语句" class="headerlink" title="if语句"></a>if语句</h2><ul>
<li>格式一</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span>(关系表达式)&#123;   </span><br><span class="line"><span class="comment">//关系表达式值为true就执行与具体，为false就不执行，继续执行if后的语句</span></span><br><span class="line">    语句体；</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<ul>
<li><p>格式二</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span>(关系表达式)&#123;   </span><br><span class="line">    语句体<span class="number">1</span></span><br><span class="line">&#125;<span class="keyword">else</span>&#123;</span><br><span class="line">    语句体<span class="number">2</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
</li>
<li><p>格式三</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span>(关系表达式)&#123;   </span><br><span class="line">    语句体<span class="number">1</span></span><br><span class="line">&#125;<span class="keyword">else</span> <span class="keyword">if</span>(关系表达式)&#123;</span><br><span class="line">    语句体<span class="number">2</span></span><br><span class="line">&#125;</span><br><span class="line">······</span><br><span class="line"><span class="keyword">else</span>&#123;</span><br><span class="line">    语句体n+<span class="number">1</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><img src="https://yolin121.oss-cn-beijing.aliyuncs.com/blogimg/Snipaste_2020-07-03_10-32-33.jpg" alt=""></p>
</li>
</ul>
<h3 id="案例"><a href="#案例" class="headerlink" title="案例"></a>案例</h3><p>输入年份判断是闰年还是平年？</p>
<ul>
<li>普通闰年:公历年份是4的倍数的，且不是100的倍数，为普通闰年。（如2004、2020年就是闰年）；</li>
<li>世纪闰年:公历年份是整百数的，必须是400的倍数才是世纪闰年（如1900年不是世纪闰年，2000年是世纪闰年）；<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">day3</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        System.out.println(<span class="string">"请输入年份："</span>);</span><br><span class="line">        Scanner sc=<span class="keyword">new</span> Scanner(System.in);</span><br><span class="line">        String ss=sc.nextLine();</span><br><span class="line">        <span class="keyword">int</span> year=Integer.parseInt(ss);  <span class="comment">//转换成整数</span></span><br><span class="line">        <span class="keyword">if</span>(year%<span class="number">400</span>==<span class="number">0</span> || (year%<span class="number">4</span>==<span class="number">0</span> &amp;&amp; year%<span class="number">100</span>!=<span class="number">0</span>))&#123;</span><br><span class="line">            System.out.println(year+<span class="string">"年是闰年"</span>);</span><br><span class="line">        &#125;<span class="keyword">else</span>&#123;</span><br><span class="line">            System.out.println(year+<span class="string">"年是平年"</span>);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h2 id="switch语句"><a href="#switch语句" class="headerlink" title="switch语句"></a>switch语句</h2><h3 id="格式"><a href="#格式" class="headerlink" title="格式"></a>格式</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">switch</span>(表达式)&#123;</span><br><span class="line">    <span class="keyword">case</span> 值<span class="number">1</span>:</span><br><span class="line">        语句体<span class="number">1</span>;</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    <span class="keyword">case</span> 值<span class="number">2</span>:</span><br><span class="line">        语句体<span class="number">2</span>;</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    ······</span><br><span class="line">    <span class="keyword">default</span>:</span><br><span class="line">        语句体n+<span class="number">1</span>;</span><br><span class="line">        [<span class="keyword">break</span>;]  <span class="comment">//可选</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
执行流程：</li>
</ul>
<p>①首先计算表达式的值</p>
<p>②依次和case后的值比较，如果有对应的值，就会执行相应的语句，在执行过程中，遇到break就会结束。</p>
<p>③如果所有的case后面的值和表达式值都不匹配，就会执行default里的语句体，然后程序结束掉。</p>
<h3 id="例题"><a href="#例题" class="headerlink" title="例题"></a>例题</h3><h4 id="1、若a和b均是整型变量并已正确赋值，正确的switch语句是-。"><a href="#1、若a和b均是整型变量并已正确赋值，正确的switch语句是-。" class="headerlink" title="1、若a和b均是整型变量并已正确赋值，正确的switch语句是( )。"></a>1、若a和b均是整型变量并已正确赋值，正确的switch语句是( )。</h4><p>A) switch(a+b);{ …… }     B) switch( a+b*3.0 )  { …… }</p>
<p>C) switch a  { …… }         D) switch ( a%b )  { …… }</p>
<h4 id="2、下面语句运行结果为：（）"><a href="#2、下面语句运行结果为：（）" class="headerlink" title="2、下面语句运行结果为：（）"></a>2、下面语句运行结果为：（）</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">switch</span> (<span class="number">5</span>) &#123;</span><br><span class="line">    <span class="keyword">default</span>:</span><br><span class="line">         System.out.print(<span class="number">5</span>);</span><br><span class="line">    <span class="keyword">case</span> <span class="number">0</span>:</span><br><span class="line">        System.out.print(<span class="number">0</span>);</span><br><span class="line">    <span class="keyword">case</span> <span class="number">1</span>:</span><br><span class="line">        System.out.print(<span class="number">1</span>);</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">    <span class="keyword">case</span> <span class="number">2</span>:</span><br><span class="line">        System.out.print(<span class="number">2</span>);</span><br><span class="line">        <span class="keyword">break</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>A) 1         B) 5         C) 0         D) 501</p>
<h4 id="3、下列语句序列执行后，k-的值是-。"><a href="#3、下列语句序列执行后，k-的值是-。" class="headerlink" title="3、下列语句序列执行后，k 的值是( )。"></a>3、下列语句序列执行后，k 的值是( )。</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> i = <span class="number">10</span>, j = <span class="number">18</span>, k = <span class="number">30</span>;</span><br><span class="line">        <span class="keyword">switch</span> (j - i) &#123;</span><br><span class="line">            <span class="keyword">case</span> <span class="number">8</span>:</span><br><span class="line">                k++;</span><br><span class="line">            <span class="keyword">case</span> <span class="number">9</span>:</span><br><span class="line">                k += <span class="number">2</span>;</span><br><span class="line">            <span class="keyword">case</span> <span class="number">10</span>:</span><br><span class="line">                k += <span class="number">3</span>;</span><br><span class="line">            <span class="keyword">default</span>:</span><br><span class="line">                k /= j;</span><br><span class="line">        &#125;</span><br><span class="line">        System.out.println(k);</span><br></pre></td></tr></table></figure>
<p>A) 31          B) 32          C) 2           D) 33</p>
<h3 id="答案"><a href="#答案" class="headerlink" title="答案"></a>答案</h3><p>1、D </p>
<blockquote>
<p>分析：A.switch(); 后面多了个分号   B.switch(常量表达式)，括号里面的表达是的值只能为:byte short int char 或者enum类型，但a+b*3.0 是double类型<br> C.()漏了</p>
</blockquote>
<p> 2、D   </p>
<blockquote>
<p>分析：switch-case中，default分支如果写在前面或中间的话要加break语句，否则执行完default分支后，才会继续往下执行，直到遇到break语句</p>
</blockquote>
<p> 3、C    </p>
<blockquote>
<p>分析：case分支中没有break;语句会继续往下执行，知道遇到break;语句，或者遇到default：分支才结束</p>
</blockquote>
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    <li class="nav-item">
      <a class="nav-item-link" href="/friends">友情链接</a>
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  </ul>
</nav>
<nav class="navbar navbar-bottom">
  <ul class="nav">
    <li class="nav-item">
      
      <a class="nav-item-link nav-item-search"  title="Search">
        <i class="ri-search-line"></i>
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      <a class="nav-item-link" target="_blank" href="/atom.xml" title="RSS Feed">
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</nav>
<div class="search-form-wrap">
  <div class="local-search local-search-plugin">
  <input type="search" id="local-search-input" class="local-search-input" placeholder="Search...">
  <div id="local-search-result" class="local-search-result"></div>
</div>
</div>
      </aside>
      <div id="mask"></div>

<!-- #reward -->
<div id="reward">
  <span class="close"><i class="ri-close-line"></i></span>
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  <div class="reward-box">
    
    <div class="reward-item">
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    <div class="reward-item">
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<script>
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    backSpeed: 40,
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    });
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<!-- Root element of PhotoSwipe. Must have class pswp. -->
<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">

    <!-- Background of PhotoSwipe. 
         It's a separate element as animating opacity is faster than rgba(). -->
    <div class="pswp__bg"></div>

    <!-- Slides wrapper with overflow:hidden. -->
    <div class="pswp__scroll-wrap">

        <!-- Container that holds slides. 
            PhotoSwipe keeps only 3 of them in the DOM to save memory.
            Don't modify these 3 pswp__item elements, data is added later on. -->
        <div class="pswp__container">
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
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        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
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                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
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                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
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            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
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            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
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            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
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            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
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        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
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<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>



<script type="text/x-mathjax-config">
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  MathJax.Hub.Queue(function() {
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      for(i=0; i < all.length; i += 1) {
          all[i].SourceElement().parentNode.className += ' has-jax';
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<script src="https://cdn.jsdelivr.net/npm/mathjax@2.7.6/unpacked/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>


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